The fuel assemblies for reactors cooled with light water and in particular the fuel assemblies for reactors cooled with pressurized water generally comprise a bundle of fuel rods each made up of a stack of pellets of fuel material inside tubular cladding, and a framework for supporting the fuel rods, the framework comprising spacer grids that are spaced apart along the rods of the bundle, guide tubes that occupy certain rod positions inside the bundle, and two end nozzles.
Inside the bundle, the fuel rods are supported in parallel dispositions by the spacer grids which are constituted by respective square-mesh arrays of square cells, each of which has a fuel rod engaged therein, or possibly a guide tube in certain positions inside the array.
The spacer grids and the end nozzles which are fixed to the ends of the guide tubes at opposite ends of the bundle of rods are square in shape so that the overall shape of the fuel assembly bundle is that of a right prism of square section, i.e. that of a rectangular parallelepiped.
The fuel rods of the bundle that are located in the outside faces of the fuel assembly that are perpendicular in pairs constitute the peripheral rods of the fuel assembly, and they
Inside the cells of the spacer grids, the fuel rods are held by springs bearing against rigid abutments, thus making it possible both to support the rods in transverse planes in a regular array and also in the axial longitudinal direction of the bundle.
In a nuclear reactor in operation, the fuel assemblies are subjected to irradiation which leads to certain modifications in the structure and size of the elements constituting the fuel assembly.
In particular, the pellets of fuel material in the rods can swell to a certain extent while the nuclear reactor is in operation. This results in the tubular cladding of the rods deforming and in an increase in the diameter of the rods.
In order to know how the fuel behaves in the nuclear reactor and in order to plan maintenance operations on the fuel assemblies, it can be important, after the fuel assembly has been in use for a certain length of time in the core of the nuclear reactor, to measure the diameter of the rods, at least in certain zones of the rods.
It is advantageous to be able to perform such measurements without taking the fuel assembly apart, but under such circumstances, diameter measurements are restricted to the peripheral rods in the fuel assembly that are accessible in the side faces of the assembly.
In addition, while the nuclear reactor is in operation, the reactor cooling water circulates a high speed in contact with the rods held in the framework by springs and dimples in the cells of the spacer grids.
As a result, the rods are caused to vibrate and are therefore subject to a certain amount of wear where they come into contact with the dimples inside the spacer grids.
It is necessary to be able to measure the wear in those segments of the rods that lie inside the spacer grids after the fuel assembly has been in use for a certain length of time in the nuclear reactor.
It is desirable to be able to perform these measurements without having to take the fuel assembly apart.
Fuel assemblies taken out from the nuclear reactor, after the reactor has stopped and cooled, are generally deposited in a deactivation pool situated in a fuel building, close to the building that houses the nuclear reactor.
Fuel assemblies are moved within the fuel pool by using a handling and hoist tool carried by a fuel pit bridge that moves over the fuel pool. In addition, an elevator which is fixed to a wall of the pool serves to move the fuel assemblies in the vertical direction.
The unloading of fuel assemblies that is performed after the nuclear reactor has cooled makes use of a loading machine which travels over a reactor pool into which the nuclear reactor vessel containing the fuel assemblies opens out, the vessel and the pool being filled with water while the fuel assemblies are being handled.
The fuel assemblies can be measured either in the fuel pool or in the nuclear reactor pool, depending on how the operations of loading and unloading the fuel assemblies are performed.
Nevertheless, a method has not been known in the past enabling the diameter of peripheral rods to be measured with very high measurement accuracy in a fuel assembly along those segments of the rod that extend between two successive spacer grids or along those segments of the rods that are received inside the spacer grids and that are subjected to wear in the reactor in operation, without it being necessary to take the fuel assembly apart.